Earlier patents, among them U.S. Pat. No. 5,287,331 & Canadian Patent 2,105,647, describe a method for manufacturing gas- and liquid-coupled ultrasonic capacitive-type transducers that are operable over a large frequency bandwidth (˜40 kHz-2 MHz). That method involves using micromachining or IC manufacturing techniques to make well-defined pitted structures in the surface of a solid or polymer material known as the backplate. The surface pits serve to trap small pockets of air when a thin metallized polymer film is placed overtop. The polymer film serves as the active element of the device i.e., generating and receiving ultrasound through vibration. In generation, a time-varying voltage V(t) is applied across the thin polymer film, grounding the outer metallized surface of the polymer film while applying V(t) to the backplate (whose surface is made conducting). This time-varying voltage (often superimposed upon a dc bias voltage), creates a time-varying electric field that drives the grounded surface of the thin film into vibration via electrostatic forces. In detection, ultrasonic waves arriving at the thin film drive the thin membrane into motion which, in the presence of an applied bias voltage across the backplate and thin-film, generates charge variations Q(t) that can be detected by charge-sensitive (or trans-impedance) amplification schemes.
Essentially, the structure is much like a large array of tiny drum-skins all vibrating in unison, with the frequency bandwidth of the transducer going up with decreasing dimensions of the backplate pits and with decreasing thickness of the thin film. In order to get high frequency responses, wide bandwidths, and high sensitivities, these earlier patents taught that it was necessary to turn away from conventional means of roughening the backplates (e.g., sanding, sandblasting, etc.) toward a more careful control over pit shapes and pit sizes using micromachining manufacturing techniques. A number of other patents have since issued with variations on this general theme, most of which employ a silicon-nitride solid membrane.
Four main problems have often appeared in implementing the micromachined approach during transducer manufacture for various markets.
First, it has often been prohibitively expensive for small companies to use micromachining techniques to prototype and run R&D efforts on new transducers. The high expense of micromachining results mainly from the need to make various photolithographic masks, etc. for the processing of micromachined structures, but also because the IC/micromachining industry is set up to serve mass-markets through the mass-production of devices (i.e., as with transistors). Basically, the set-up costs are typically high with micromachining, though the per-unit costs can be low at high volume. Naturally not all markets are large enough to justify high numbers of units and therefore a less expensive means of fabricating capacitive ultrasonic transducers for use in fluids (i.e., gas and liquids) would be advantageous for satisfying a wider variety is of markets.
The second problem has been that not all markets and applications for capacitive transducers actually require the highest-bandwidths that result from the micromachining of transducers. This leads to unnecessary expenses for micromachining of transducers, when a less-involved fabrication method capable of providing somewhat reduced, though still sufficient, acoustic performance would suffice.
The third problem has resulted from attempts to create roughened backplates having 3-dimensional top surfaces (e.g., spherical) so as to gain control over resultant ultrasonic field shapes and so create such devices as focussed transducers. Such transducers with 3-dimensional top surfaces are not easily created via micromachining techniques at present, because the IC/micromachining industry has been developed predominantly for use on planar 2-dimensional surfaces (such as integrated circuits). Thus, the provision of a method that would allow an ease of integrating 3-dimensional or curved backplate elements would be particularly advantageous (regardless of whether those elements are created by micromachining or conventional means).
The final problem has been that, regardless of whether micromachining or other methods are employed for the creation of backplates, careful attention must be directed toward: (a) effective electromagnetic shielding and packaging issues for the backplates; and (b) an ease of integration with associated discrete electronics (both through-hole, and Surface Mount Technology or SMT). Shielding and packaging issues are of particular concern when mounting numerous backplates within a single housing (e.g., as in the creation of multi-element capacitive transducers), but also when the total volume (or size) of a transducer assembly is a concern, as it often is for various markets. Thus, it would be a significant advance to provide a more convenient fabrication method, if that method were to ease the effective electromagnetic shielding, packaging and electronic integration of capacitive ultrasound transducers.